In the art of magnetic resonance examination, the birdcage resonator, also known as birdcage coil, is a well-known volume radio frequency coil design for generating a radio frequency magnetic excitation field B1 to be applied to nuclei of or within a subject of interest for magnetic resonance excitation, wherein the subject of interest is positioned at least partially within the birdcage coil, which in turn is arranged within a static, homogeneous magnetic field B0 arranged substantially perpendicular to the radio frequency magnetic excitation field B1.
In the art, birdcage resonators are known to serve as radio frequency transmit coils and/or radio frequency receive coils. They are usually operated in resonance at a radio frequency corresponding to the Larmor frequency, which depends on the strength of the static magnetic field B0 in the gyromagnetic magnetic ratio of the species of nuclei under consideration.
U.S. Pat. No. 4,680,548 describes the volume radio frequency coil design, later referred to as the “birdcage coil” for its appearance, as a magnetic resonance radio frequency coil having a pair of conductive loop elements spaced along a common longitudinal axis. Each of the loop elements includes a plurality of serially-connected capacitive elements spaced along the loop peripheries. A plurality of axial conductive elements (commonly referred to as “rungs”) electrically interconnect the conductive loop elements at points between adjacent ones of the serially connected capacitive elements. In the high-pass embodiment of the radio frequency coil, the axial conductive segments may be wires, conducting tubes or flat conductive tapes whose inherent inductance is needed for proper coil operation. A band-pass embodiment of the coil is realized by including capacitive elements in each of the axial conductive segments. Birdcage coils are known to have as many resonant modes as there are radial or axial conductive segments. The preferred excitation mode for the birdcage coil is the one in which a generated radio frequency magnetic excitation field B1, when operated as a transmit coil, is as homogenous as possible. This is the case for resonant modes whose current distribution in the rungs is proportional to sin θ or cos θ, respectively, wherein θ denotes the azimuthal angle measured circumferentially about the birdcage coil axis.
U.S. Pat. No. 4,680,548 further describes to operate the birdcage coil in a quadrature excitation mode in which the birdcage coil transmits a circularly polarized radio frequency magnetic field, known to maximally interact with nuclei spins. To this end, the birdcage coil is excited at two input capacitors located at right angles relative to one another, for instance along the circumference of one of the conductive loop elements, by two radio frequency sources that are electrically 90° out of phase relative to one another. In the case of quadrature excitation, the magnitude of the currents in each rung is equal while the relative phase angle increments in a linear manner with the azimuthal angle θ.
It is further known that for static magnetic fields B0 of large field strength such as 3 T or higher, dielectric-based standing wave mechanisms related to the subject of interest severely affect the homogeneity of the radio frequency magnetic excitation field B1. One option for improving homogeneity of the radio frequency magnetic excitation field B1 in a birdcage coil in this case has been suggested in U.S. Pat. No. 6,043,658. Herein, a band-pass birdcage coil is described to be operated in a degenerate mode of operation in which all resonance modes and all resonance frequencies coincide, which is achieved by selecting specific values for the capacitances in the rungs and the conductor loops, respectively. Individual meshes (each mesh comprising two neighboring conductors and the sections of the two conductor loops which are situated in between) of the birdcage coil are then decoupled from one another, and thus operation as a coil array is enabled, wherein each coil can independently be driven by a radio frequency amplifier in order to optimize the uniformity of the radio frequency magnetic excitation field B1 (so-called radio frequency “shimming”).
In international application WO 2014/053289 A1, a band-pass birdcage coil is described wherein the ratio of ring to rung capacitance values is chosen to realize N resonant modes, each tuned to the same resonance frequency, whereby, for each resonant mode, individual meshes of the birdcage resonator are not substantially electrically decoupled from each other. The radio frequency magnetic excitation fields B1 generated in the different resonant modes are then not equal in spatial distribution to those of the degenerate mode of operation described in U.S. Pat. No. 6,043,658 and allow for linear combination such that a power-efficient, uniform, circularly polarized field distribution of the radio frequency magnetic excitation fields B1 can be accomplished.
The international application WO02/095435 concerns an arrangement of generating RF fields in the examination volume of an magnetic resonance examination system. Control of the RF field distribution is achieved by individual selection of the phase and amplitude of the RF feeding to each of the resonator segments. The international application WO2006/076624 discloses a birdcage coil which has a ring resonator provided to the end-rings of the birdcage resonator. A fixed 90° quadrature splitter creates a travelling wave about the ring resonator to drive the birdcage coil in the quadrature mode.